The present invention relates to a speech coding system for high quality coding speech signals at a low bit rate, particularly a bit rate of 8 kb/sec or less, with a comparatively small amount of operations.
As a prior art speech coding system for vector quantizing an excitation signal with an excitation codebook, a CELP system is well known. This system is disclosed in a treatise by M. R. Shroeder and B. S. Atal entitled "Code-Excited Linear Prediction (CELP): High-Quality Speech at Very Low Bit Rates", Proc. ICASSP for Acoustic, Speech and Signal Processing, 1985, p--p 937-940 (literature 1). Also, as a CELP system having an adaptive codebook, a CELP system is well known, which is disclosed in a treatise by W. B. Kleijn et al entitled "Improved Speech Quality and Efficient Vector Quantization in SELP", Proc. ICASSP for Acoustic, Speech and Signal Processing, 1988, p--p 155-158 (literature 2). In these CELP systems, optimal codevectors are searched from excitation, adaptive and gain codebooks to minimize the perceptually weighted square distance between the input and coded speech signals for each subframe length. However, since the coding is done for each subframe, distortion is liable to result at the block boundary in the block coding, and therefore sufficiently satisfactory speech sound quality can not be obtained. To alleviate the distortion at the block boundary of the block coding, a speech coding system has been proposed in a treatise by LeBlanc et al entitled "Structured Codebook Design in CELP". International Mobile Satellite Conference, 1990, p--p 667-672 (literature 3). In this system, an optimal codevector is searched from an excitation codebook to minimize the perceptually weighted square distance between two signals. The first signal is obtained by connecting the next subframe input speech signal for a predetermined length called overlap length to the present subframe input speech signal. The second signal, is obtained by connecting an influence signal of a coded speech signal having a length corresponding to the overlap length to the trailing end of the coded speech signal.
In the prior art systems noted above, the distortion at the block boundary of the block coding still cannot be sufficiently reduced although the distortion can be reduced to a certain degree.